The present invention monitors an induction motor while it is operational, and detects and locates rotor faults in real time without interfering with the operation of the motor.
In this specification, the term "rotor fault" means a break or partial break in any of the rotor bars, or in the end rings, of the rotor in an induction motor. Rotor faults, although not as common as bearing or insulation faults, can cause catastrophic destruction of an induction motor. Especially in large motors, rotor faults can be extremely dangerous.
The electric power industry, among others, has tried for many years to develop reliable methods of detecting rotor faults well before such faults result in destruction of the motor or damage to associated systems (e.g., caused by motor vibration). To the extent possible, the ideal rotor fault detector will be extremely reliable (i.e., it will find all faults, and will have a low false alarm rate), will not require interruption of the motor's operation, and will be inexpensive to build and operate.
The prior art includes numerous methods of detecting rotor faults, also known as "cage faults". Most require one or more of the following actions: disassembly of the motor, motor shut down, and/or special connection of instrumentation inside the motor. For instance, the growler method uses an electromagnet coupled to the rotor surface which emits a loud noise when it spans an open rotor bar. This requires disassembly of the motor.
"Single phase" testing requires disconnecting one phase of the motor's power supply, and monitoring the current drawn while exciting the remaining terminals at low voltage and rotating the rotor slowly, by hand. If there is a broken bar the current drawn will vary with rotor position. While sensitivity is good--a broken bar is usually clearly evidenced by a current variation of over five percent--the motor must be taken out of service and one phase disconnected. Further, the low voltage power requirement is considered to be a safety hazard by many utility companies.
The present invention detects rotor faults without interrupting motor operation, by analyzing two signals which are available external to the motor while the motor is running: the motor's line current and the axial flux signals external to the motor. Therefore motor shutdown is not required, not even to install a flux sensor. The underlying theory of operation is that certain "rotor fault harmonics" of the motor's line frequency (actually certain sidebands of nf.sub.o, where n is an integer and f.sub.o is the fundamental line frequency of the motor's power source) will be markedly different for motors with normal rotors and for motors with at least one rotor fault. Another part of the theory of operation is that measurement of a motor's axial flux signal, at the motor's slip frequency, can determine the presence of an end ring break.
It is therefore a primary object of the present invention to provide a nonintrusive system and method for detecting rotor bar faults in induction motors.
Another object and feature of the present invention is autonomous operation. The invention is designed so that the user neither needs to understand its operation, nor needs to perform any functions which would require knowledge of data analysis, or even induction motors.